In order to protect occupants in a vehicle and pedestrians, various airbag devices have prevailed. For example, in order to protect an occupant seated on a seat in a vehicle, a knee airbag device is used. The knee airbag device is mounted on an instrument panel and disposed in front of the knee part of the occupant. In an emergency or a collision of the vehicle, the knee airbag device inflates and deploys an airbag and moves a protective panel for knees by the airbag to the knee part of the occupant. The knee part of the occupant is brought into contact with the protective panel, and movement of the knee part is prevented. As a result, the knee part is protected. As this knee airbag device, an airbag device provided with an inflatable bag main body and a surface member covering the bag main body is known (See Patent Document 1).
This prior-art airbag device inflates the bag main body between the instrument panel and the surface member. The surface member is moved by the bag main body, and the knee part of the occupant is protected by the surface member. However, with this airbag device, since the bag main body is brought into contact with the whole back surface of the surface member and deployed into a flat rectangular shape, a volume of the bag main body becomes large. As a result, time until the bag main body is fully inflated and deployed tends to become long. Along with this tendency, a moving speed of the surface member also becomes slow, and thus moving time of the surface member might become long. The moving time of the surface member is time until the surface member moves to a position where the surface member protects the knee part of the occupant (position the closest to the knee part). Therefore, from the viewpoint of protecting the knee part of the occupant early and reliably, improvement is required.